In the field of medicine, a study of imaging of function information as physiological information on a biological body is recently conducted. Photoacoustic imaging (PAI) is one of imaging techniques of the function information.
In the photoacoustic imaging, pulsed light generated from a light irradiation unit (light source) is applied to an object. When energy of the applied light that has propagated and diffused in the object is absorbed by a light absorber in the object (e.g., a blood vessel or a skin), an acoustic wave is generated by a photoacoustic effect (hereinafter referred to as a photoacoustic wave). A reception signal converted from the photoacoustic wave by a conversion element is subjected to analysis processing by an information processing apparatus, and an optical characteristic distribution inside the object is thereby acquired. By converting the optical characteristic distribution into image data and displaying the image data, information useful for diagnosis or the like is obtained.
The optical characteristic distribution includes the distribution of a sound pressure generated by light absorption (initial sound pressure distribution) and an absorption coefficient distribution of the light. By applying a plurality of pulsed light beams having different wavelengths to determine the absorption coefficient of the light beam of each wavelength, a concentration relating distribution of a substance that exists in the object (the distribution of values relating to the concentration of the substance) is obtained.
The concentration relating distribution includes the distribution of the content of oxyhemoglobin to total hemoglobin in blood, i.e., an oxygen saturation distribution in blood as disclosed in NPL 1. This is a value determined based on the result of acquisition of respective contents by using a difference in light absorption spectrum between deoxyhemoglobin and oxyhemoglobin to compare spectrums measured with different wavelengths. By determining the oxygen saturation at each position in the object, the oxygen saturation distribution is acquired. In addition, in NPL 1, the oxygen saturation of not less than 0% and not more than 100% is displayed with a hue corresponding to the value of the oxygen saturation.